The original Griffith body does not exist anymore, it was tortured and mutilated. His consciousness was transformed into Femto, which is a Godhand (ie. not human, exists on a different plane than humanity). He wasn't reborn into Femto, he was transformed.Maybe one day I'll read the manga. Nina was such an insufferable character jeez.
I just finished this show. Although there's a LOT I could not follow, what I least understand is Griffith's rebirth. Didn't that already happen at the end of the Golden Age arc? What am I watching here?
In order for him to be Griffith in the human world, he needs a physical body that he can manifest himself into (ie. skin suit). He can't exactly go around in Femto form as it's not exactly a human form (and it's two different planes of existence). All of this needed a very complex ritual for it to happen. This is again a transformation, not really a rebirth. They are all the same person in mind but the physical entity is different in all 3 cases (so goes from Physical to Vortex to Interstice presumably).
TLDR; Read the manga. There are far more details, nuances and subtleties to this arc that were lost in this poor adaptation. Also read the lost chapter where
Griffith has the conversation with the Idea of Evil.
Also this is the realm of existence I was referring to: